1. Field of the Invention
The present invention relates to a gas sensor, and more particularly to a gas sensor for sensing fruits or vegetables and a method of manufacturing the same.
2. Discussion of the Related Art
An effective sensor for sensing an inherent smell component of food (such as fruits and vegetables) is yet to be developed. Among the sensors in existence, the sensitivity for sensing the inherent smell component of the food had been too low to sense fruits and vegetables being put into or out of a container. Already-developed sensors merely sense the gas produced when the food (fruits and vegetables) decays or its freshness degrades.
Generally, when vegetables such as radish, Welsh onion, carrot, or lettuce are put in a storage compartment, the vegetables produce reducing gases, such as sulfuretted hydrogen (H.sub.2 S), methyl-mercaptan (CH.sub.3 SH), dimethyl-sulfide ((CH.sub.3).sub.2 S), and dimethyl-disulfide ((CH.sub.3).sub.2 S.sub.2), as the storage time grows.
If the vegetables are stored for a long time as described above, the sulfuric compound gases (reducing gases) are generated due to the degraded freshness. Thus, these gases are bonded with oxygen ions adsorbed on the surface of the gas sensor to generate conducting electrons as in the following reaction formula, thereby changing an electric conductivity on the surface of a sensing layer of the gas sensor: EQU R+O=RO+e.sup.-
Since the reaction of this formula is active in the vicinity of a metal catalyst on the surface of a sensing layer of the gas sensor, it is very important as to which catalyst of a certain component is added and how much is added. Conventionally, a palladium (Pd) catalyst of 1 wt % is added, and SnO.sub.2 is utilized as the sensing layer.
The above-mentioned conventional gas sensor for sensing gases produced due to the degraded freshness from prolonged storage will be described with reference to accompanying drawings. FIGS. 1a-1c is a view showing a structure of a conventional gas sensor for sensing the freshness of food, and FIG. 2 shows a process of manufacturing the gas sensor for sensing the freshness of food.
In manufacturing a gas sensor for sensing the freshness of the food, SnO.sub.2 powder is adequately mixed with Pd powder, which is finely ground to be subjected to a heat treatment at 700.degree. C. for about 5 hours. Then, the heat-treated powder mixture is finely ground further, and mixed with an organic material consisting of ethyl-cellulose and alpha-terpinol to create a paste. The mixture in the paste is coated to have a proper size and thickness on an electrode plane of alumina substrate 1 formed with electrodes 2 and a heater 3 via a screen printing method, thereby forming a sensing layer 4.
Thereafter, a drying process is performed; the gas sensor is sintered at 600.degree. C. for 10 minutes; a lead wire is attached; and a packaging process is performed. Thus, the gas sensor is completed.
FIGS. 1a-1c illustrate a situation where the heater is formed on the bottom plane. FIG. 3 is a circuit diagram for showing the operation of the conventional gas sensor. An operation of the conventional gas sensor for sensing the freshness of the food is carried out as follows.
The gas sensor reacts with the sulfuric compound to change electric conductivity. At this time, a resistance variation Rm is supplied to a microprocessor 5 as a voltage variation value in accordance with a resistance dividing ratio with a fixed resistor R.sub.L.
In this operation, microprocessor 5 compares the input voltage value with a pre-programmed value to determine the freshness of the vegetables in the storage compartment. Then, once the freshness begins degrading, the rate of freshness degradation is maximally retarded by a temperature control, a humidity control, a pressure control, a quantity control of oxygen/ozone, a quantity control of an anion, etc.
However, the conventional gas sensor and the freshness maintaining apparatus for using the same have the following problems. First, the conventional gas sensor which senses the freshness of the food, such as fruits and vegetables, has low sensitivity in sensing the inherent smell component of the fruits and vegetables, so that it is not distinguishable whether the fruits or vegetables are being put into or being put out from the storing compartment. Moreover, since the putting in or putting out of the fruits and vegetables cannot be distinguished, it is controlled by recognizing that the food, such as the fruits or vegetables, in the storage compartment is as good despite the fact that there is no food in the storing compartment. Consequently, the power consumed by a refrigerator is unnecessarily increased.